1. Technical Field
The present invention relates to a liquid developer used in electrophotographic type image forming apparatuses used in photocopiers, printers and the like, and an image forming apparatus using the same.
2. Related Art
Image forming apparatuses using liquid developers are characterized in that, even in the case of using fine particles, high precision images can be formed without having problems caused by scattering of the particles to the outside of the apparatus, and the like. An electrophotographic type image forming apparatus using a liquid developer uses a developer in which colorant particles, or a toner containing colorant and resin as the main components is dispersed in a carrier liquid, and an electrostatic latent image formed on a photoreceptor by exposure is developed using a liquid developer.
After developing, the obtained latent image is transferred onto a recording medium such as paper, and fixed to form an image.
Liquid developers have been generally using petroleum-based volatile hydrocarbon solvents as the carrier liquid. However, although the volatile hydrocarbon solvents are stable materials with low electric conductivities, it is required to volatilize or evaporate the volatile hydrocarbon solvent of the carrier liquid upon fixing the toner transferred or recorded on the recording medium. When evaporated volatile hydrocarbon solvents are discharged, the solvents would cause environmental contamination.
Thus, to give consideration to the environment of use, it is necessary to provide in the vicinity of the fixing apparatus, means for recovering the carrier liquid which has turned into a gas phase. However, an increase of the size is now indispensable for such image forming apparatuses because of the presence of the means for recovering carrier liquid, and thus it is disadvantageous for the image forming apparatuses to be miniaturized.
Furthermore, it has been also proposed to prevent evaporation or volatilization of the carrier liquid by using non-volatile silicone oil or liquid paraffin as the carrier liquid. However, since the chemical properties of these materials as the carrier liquid are stable, the carrier liquid tends to remain on the recording medium even after the fixing process. As a result, there have been problems that the texture of the printing quality is deteriorated, or the presence of the carrier liquid on the paper surface results in deterioration of impressibility, deterioration of the writing characteristics of writing instruments using water-soluble inks, and the like.
Also, during the manufacturing process for the liquid developers according to the related art, while the respective components are used in a state of being dispersed in a non-aqueous, non-polar solvent as the carrier liquid, the properties of the non-polar solvent would cause problems such as large-sized apparatuses, deterioration of the quality of recorded materials, poor storage stability of the liquid developer, and the like.
Meanwhile, it has been repeatedly proposed to use vegetable oils, instead of volatile hydrocarbon organic solvents, as the carrier liquid. For example, JP-A-2000-19787 suggests that by using vegetable oils as the carrier liquid for liquid developers, an odorless carrier liquid with small particle size, having enhanced image density, resolution and fixability, can be obtained.
In the case of preparing a positively charged liquid developer using a vegetable oil as the carrier liquid, it is essential to use a charge controlling agent (CCA) for the process of positive charging. If the amount of the charge controlling agent used is increased, the storability of the dispersion becomes unstable. In particular, when a liquid-state metal soap is used, there is a tendency that the viscosity of the dispersion is increased during long-term storage, and it becomes difficult to expect successful accomplishment of the function from the liquid developer.
On the other hand, when vegetable oils are used as the carrier liquid, there is obtained a feature that oxidative polymerization of the unsaturated bonds present in the vegetable oils brings about rapid and stable image formation of the images transferred onto paper, while there is also a problem that occurrence of the oxidative polymerization at a high rate may lead to a decrease in fluidity during storage, and subsequent deterioration of the carrier liquid.
For example, JP-A-2003-335998 suggests adding an antioxidant to a vegetable oil having an oxo value of 100 to 150, for an emulsion ink useful for stencil printing, which uses a vegetable oil as a dispersion medium, and it is suggested to add bisphenol-based antioxidants, sulfur-based antioxidants, phosphite-based antioxidants and the like as the antioxidant for the purpose.
However, for the use as the carrier liquid of liquid developers, there has been a demand for materials having excellent image forming properties as for the liquid developer, for example, in terms of charging properties, in addition to the problems such as oxidation prevention, polymerization prevention and the like. However, antioxidants have never been examined in relation to the image forming properties and the like.